The 3rd-Generation Partnership Project (3GPP) has recently developed specifications, applicable to Release-7 Universal Terrestrial Radio Access (UTRA) systems, for delivering so-called Multimedia Broadcast Multicast Services (MBMS) using a single-frequency network (SFN). MBMS over SFN (MBSFN) provides significantly higher spectral efficiency compared to the MBMS approach in earlier systems (e.g., Release 6 systems), and is primarily intended for broadcasting mobile television services that demand high bit-rates on carriers dedicated to MBMS. Since MBMS services are broadcast only, MBSFN is inherently suited for transmissions in unpaired frequency bands.
With SFN transmissions, multiple base stations transmit the same waveform at the same time. A mobile terminal can receive signals from two or more of these base stations and treat the received signal as if it was transmitted by a single base station serving a large cell. For UTRA systems, SFN transmission implies that a cluster of time synchronized base stations (Node B's, in 3GPP terminology), transmit the same data, using the same channelization and scrambling codes.
Mobile terminals developed for use in Wideband Code-Division Multiple Access (W-CDMA) systems generally use a continuously transmitted, code-multiplexed pilot signal (known as the common pilot channel, or CPICH, in 3GPP specifications) for channel estimation. Although a code-multiplexed pilot channel performs well in unicast radio environments, the MBSFN channel has a much larger delay spread and thus a larger number of paths to estimate in the radio receiver. Accurate channel estimation requires long averaging over many slots. This is difficult to realize in situations where a small duty cycle is used, i.e., where a mobile terminal's receiver is only operational during intervals that are as short as possible. Thus, improved pilot signals are needed.